1. Field of the Invention
The present invention relates to a method of producing a coating composition for magnetic recording, and particularly to a method of producing a magnetic recording coating composition which is characterized by the manner of dilution of a kneaded mixture of magnetic layer forming materials, and the like.
2. Description of the Related Art
A coating type magnetic recording media such as magnetic tapes, magnetic disks, etc., are heretofore produced in a manner in which a magnetic coating composition having ferromagnetic powder dispersed in a resin (binder resin) dissolved in an organic solvent is applied onto a running belt-like non-magnetic support to thereby provide a magnetic layer on the support; orientation treatment, drying/solidification treatment and surface treatment are applied to the magnetic layer successively; and the support with the magnetic layer is cut or punched finally.
Generally, the magnetic coating composition is prepared by dispersion of ferromagnetic powder and additives such as an abrasive material, an antistatic agent, a lubricant, a dispersant, a hardener, etc. in a binder resin. A sophisticated technique is required for mixing relatively fine needle-shaped ferromagnetic powder particles with anisotropy evenly in a binder resin to thereby produce a highly dispersed magnetic coating composition. Therefore, various proposals have been made in the conventional art.
Unexamined Japanese Patent Publication (Kokai) No. 4-47526 discloses a method for producing a magnetic coating kneaded mixture, which comprises a wetting step, a first-half kneading step, a second-half kneading step, and a dilution step. In the wetting step, a magnetic layer forming material provided as a powder and containing ferromagnetic powder, an antistatic agent, and an organic solvent, is added into a kneader and mixed/wetted. It has been suggested that the amount of the organic solvent should be selected to have a predetermined amount. In the first-half kneading step in which a binder resin solution is added continuously into the kneader to prepare a kneaded mixture, it has been suggested that the binder resin and the mixing solvent be selected to have predetermined amounts respectively.
In the second-half kneading step, an organic solvent is added to the kneaded mixture to make the solid content proportion of the kneaded mixture moderate to thereby provide the kneaded mixture as a large lump, and it has been proposed that the addition of the organic solvent be adjusted on the basis of electric power consumed by the kneader. Because the kneaded mixture is provided as a large lump in the aforementioned manner, a large shearing force can be applied to the kneaded mixture by means of the kneader, so that good kneading can be performed.
In the dilution step in which an organic solvent is further added to the kneaded mixture to dilute the kneaded mixture so that the kneaded mixture has a viscosity adapted for the next dispersion step, there is a proposal that the rate and amount of addition of the organic solvent are selected to be predetermined values respectively.
It is said that a magnetic coating composition which is highly dispersed can be obtained by the aforementioned configuration.
Recently, however, high recording density has been required in magnetic recording media, so that a shorter recording wavelength is used. For shorter recording wavelengths, the thickness of the magnetic layer becomes problematic. Since both the self-demagnetization loss at the time of recording and thickness loss at the time of reproducing increase if the thickness of the magnetic layer increases, the thickness of the magnetic layer must be reduced. In order to reduce the thickness of the magnetic layer while securing the surface characteristic of the magnetic layer, a method is therefore employed in which a non-magnetic coating composition is applied onto a support to provide a non-magnetic layer on the support and then a thin magnetic layer is provided on the non-magnetic layer.
The non-magnetic coating composition is prepared by dispersion of non-magnetic powder and additives such as an abrasive material, an antistatic agent, a lubricant, a dispersant, a hardener, etc. in a binder resin.
On the other hand, in order to produce a magnetic recording medium in such a manner that a non-magnetic coating composition is applied onto a support to thereby form a non-magnetic layer on the support and then a thin magnetic layer is formed on the non-magnetic layer, not only the magnetic coating composition but also the non-magnetic coating composition must be produced. Generally, the non-magnetic coating composition is produced in the same manner as that described above for producing the magnetic coating composition. In the case of producing the non-magnetic coating composition, at the time of dilution, however, a large lump of a kneaded mixture comes apart into a plurality of small lumps of the kneaded mixture due to solvent shock. Accordingly, kneading becomes insufficient so that the degree of dispersion of the non-magnetic coating composition cannot be enhanced consequently. If the degree of dispersion of the non-magnetic coating composition is low, a smooth interface between the non-magnetic layer and the magnetic layer cannot be provided. This adversely influences the magnetic layer to obstruct the improvement of the electromagnetic transducing characteristic of the magnetic recording medium consequently.